Regulation of hepatic heme synthesis by drugs, bile acids and nutrition : a transcriptional network regulating [delta]-aminolevulinic acid synthase 1 (ALAS1)

ALAS1 is the rate limiting enzyme of heme synthesis. It is highly inducible in liver in cases of increased heme demand, such as drug metabolism or in inducible hepatic porphyrias. The clinical hallmark of these rare genetic disorders of enzymes of heme synthesis are neuropsychiatric attacks precipitated by drugs, hormones and fasting. The xenosensing nuclear hormone receptors CAR and PXR have been previously shown to mediate ALAS1 induction by classical inducer drugs (Podvinec et al. PNAS, 2004). The molecular details of the action of other precipitating factors, however, have not been elucidated so far. Studying the molecular mechanism of the fasting response as well as the involvement of additional nuclear hormone receptors in the transcriptional regulation of ALAS1 was the aim of this study. We show that ALAS1 is regulated by the peroxisome proliferator-activated receptor γcoactivator α (PGC-1α) via an insulin sensitive FOXO1 site within the promoter of ALAS1. In vivo studies confirm that ALAS1 is induced at the transcriptional level upon fasting. This effect is lost in liver specific PGC-1α knock-out animals, clearly demonstrating that PGC-1α is the master regulator of the fasting response. Interestingly, induction of ALAS1 by classical inducer phenobarbital is preserved, indicating that PGC-1α is not essential for the drug mediated induction of the gene. In an attempt to identify additional factors regulating ALAS1, we have analyzed by phylogenetic footprinting the genomic sequence of human ALAS1. At minus 14kb of the transcriptional start site a novel response element for the bile acid receptor FXR, a nuclear receptor involved in lipid and glucose homeostasis as well as detoxification, was identified. Based on the molecular characterization of the FXR response element, and functional data in primary human hepatocytes and in mice as well as on genomic sequence analysis, we demonstrate that the response of ALAS1 to bile acid is unique to primates. In addition, multiple enhancer modules identified by cross species sequence comparison were shown to be activated by hepatocyte nuclear factor 4α (HNF4α). Chromatin immunoprecipitation confirmed the binding of this factor in a native chromatin context. We therefore hypothesize that the liver enriched transcription factor HNF4α is required for basal expression and liver specific induction of ALAS1. In conclusion, in the present work we describe and discuss a complex regulatory network consisting of FOXO1, the nuclear receptors CAR, PXR, FXR and HNF4α as well as the coactivator PGC-1α, which mediate the transcriptional regulation of ALAS1 to various stimuli, such as fasting, or to the exposure to endo- and xenobiotics.